THE LARVA OF ASTERIAS VULGARIS. 115 



distinctly marked difference from the rest of the ectoderm. 

 In the young larva all the ectodermal cells are cubical and 

 ciliated ; with the loss of the general ciliation the ectodermal 

 cells become flattened and more irregular in outline, except in 

 the region where the cilia persist as the ciliated bands. Here a 

 great change has taken place. The cells become very much 

 crowded together. The nuclei appear to be restricted to the 

 deeper part, while the external part is formed of granular or 

 finely fibrillated substance. The external surface is thickly 

 ciliated, with many cilia on the free surface of each cell. 



Further History of the Enteroccels. — The appearance 

 of a larva six days old, seen from the dorsal side, is shown in 

 fig. 20. The anus no longer has a terminal position (fig. 11), 

 but by the bending of the intestine it has come to lie some 

 distance forwards upon the ventral surface. The oesophagus, 

 formed by the union of an entodermal evagination of the 

 archenteron with the stomodueal ingrowth of the ectoderm 

 (fig. 11), has elongated somewhat. The oral depression has 

 become more pronounced, and the circumoral ciliated band has 

 become divided at the apical pole, in the manner described 

 above, into the two bands which characterise the Bipinnaria. 

 Before this time the enteroccels have elongated somewhat. 

 The right water-pore has disappeared, and ouly the left 

 enterocoel has an opening to the exterior. At a considerably 

 later stage (fig. 30) the enteroccels have extended anteriorly 

 as two cylindrical tubes, nearly parallel and slightly dorsal to 

 the oesophagus. The anterior end of the tube is solid, and its 

 tip is formed of branching mesenchyme-like cells. Posterior to 

 this the walls are thin, and formed of flattened cells and 

 mesenchymatous muscle-fibres (fig. 32). In a stage a little 

 older than fig. 30 (fig. 28) these two cylindrical tubes have 

 extended further forwards and into the preoral lobe. The 

 right and left enteroccels have united just in front and dorsal 

 to the mouth. At the point of union the tubes are still solid 

 (fig. 33). At a little later stage the cavities have united, and 

 the two enteroccels stand in open communication with each 

 other by their union in the preoral lobe. They later grow 



